There are three types of best friends. Truthfully, there are more, but only three matter here. These types are the Underappreciated, the Loyalist, and the Secret Keeper. The BBC show, Merlin provides good examples for these types of best friends.
Merlin, the title character, is the best description of the Underappreciated. He is a sorcerer in a place where magic is outlawed. So does he leave town so he can be himself and, more importantly, alive? No. He becomes Prince Arthur's servant and saves Arthur's life…often. Arthur doesn't know this, so he doesn't thank his best friend, but being Arthur, he probably wouldn't thank Merlin even if he did know.
Despite this, Arthur is a good friend. He is the Loyalist. He may be the prince of Camelot, but King Uther is a very biased man. Arthur believes Merlin when he says the kingdom is in danger and stands by his servant, which puts him at odds with the King more often than not. However, Arthur is loyal to a fault.
Merlin's mentor, Gaius, is also loyal, but he is better known as the Secret Keeper. Gaius is the only one in Camelot who knows that Merlin can do magic and Gaius will do anything to keep Merlin's secret safe, even be burned at the stake as a sorcerer.
Knowing the different types of best friends can keep the peace between them and maybe even save a life…more than once.